Code
//聲明一個double變量total來計算總計
double total = 0.0d;
private void btnOk_Click(object sender, EventArgs e)
{
    //聲明一個double變量totalPrices來計算每個商品的單價（txtPrice）*數量(txtNum)后的合計
    double totalPrices=Convert.ToDouble(txtPrice.Text) * Convert.ToDouble(txtNum.Text);
    //將每個商品合計計入總計
    total = total + totalPrices;
    //在列表框中顯示信息
    lbxList.Items.Add("單價："+txtPrice.Text+" 數量："+txtNum.Text+" 合計："+totalPrices.ToString());
    //在lblResult標簽上顯示總計數
    lblResult.Text = total.ToString();
}

Code
double total = 0.0d;
private void btnOk_Click(object sender, EventArgs e)
{
    double totalPrices=0d;
    //cbxType是一個下拉選擇框，分別有“正常收費”、“打8折”、“打7折”和“打5折”
    switch(cbxType.SelectedIndex)
    {
        case 0:
            totalPrices = Convert.ToDouble(txtPrice.Text) * Convert.ToDouble(txtNum.Text);
            break;
        case 1:
            totalPrices = Convert.ToDouble(txtPrice.Text) * Convert.ToDouble(txtNum.Text) * 0.8;
            break;
        case 2:
            totalPrices = Convert.ToDouble(txtPrice.Text) * Convert.ToDouble(txtNum.Text) * 0.7;
            break;
        case 3:
            totalPrices = Convert.ToDouble(txtPrice.Text) * Convert.ToDouble(txtNum.Text) * 0.5;
            break;

    }
    total = total + totalPrices;
    lbxList.Items.Add("單價：" + txtPrice.Text + " 數量：" + txtNum.Text + " "+cbxType.SelectedItem+ " 合計：" + totalPrices.ToString());
    lblResult.Text = total.ToString();
}